Why is rolling friction less than sliding friction?

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Sliding or kinetic friction is the result of force of interaction between the particles that make up the two surfaces that are in contact when they move against each other. It is the value of the force of friction after the bodies have started to move and is less than the force of static friction. The force of kinetic friction is given by the formula Fk*N, where N is the normal force and Fk is the coefficient of kinetic friction. Fk usually has values around 0.3 .

Rolling friction is considerably less than sliding friction as there is no work done against the body that is rolling by the force of friction. For a body to start rolling a small amount of friction is required at the point where it rests on the other surface, else it would slide instead of roll. This is provided by the static friction between a small part that is in contact with the surface it would roll on. The coefficient of rolling friction has values in the range 0.02-0.06 .

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Sliding (or kinetic) friction occurs when two surfaces rub against each other. All the small protrusions from both surfaces impede the motion between them and must be overcome. Rolling friction (or rolling resistance) also occurs between two surfaces, but it applies to a spherical or cylindrical object, such as a ball or wheel, moving over a surface. Non-elastic effects are the main cause of this friction, in that not all the necessary energy for movement (or deformation) is recovered when the object is removed. The slippage between the surface and the object also dissipates energy; this is the aspect that is actually friction. Because the points of contact are much smaller in rolling friction, the resistance between them is less than between two large surfaces in contact.

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